The replacement of polymeric foam insulators using carbon fluorocarbons (CFC's) as the foaming agent and commonly utilized for thermal insulation in doors and walls of refrigerators, freezers, and the like, is becoming of increasing importance from an environmental standpoint due to the deleterious impact CFC's have upon the ozone layer protecting the earth from dangerous levels of ultraviolet radiation. Recent efforts in this task involve the formation of the so-called powder evacuated thermal insulating panels which are formed by enclosing thermal insulating powders in an evacuated, substantially gas impervious envelope and placing the envelope within a suitably configured support panel. An example of such a powder evacuated panel and the thermal insulating powders used therein is described in U.S. Pat. No. 4,636,415. In this patent, precipitated silica powders formed by the interaction of an alkali water grass and a mineral acid followed by spray drying and milling are confined in an evacuated envelope and the powders then compressed to a desired density to form a relatively rigid, board-like structure for placement in the insulating panel. Another recent development in powder evacuated panels is described in U.S. Pat. No. 4,681,788 and involves the blending of precipitated silica such as set forth in U.S. Pat. No. 4,636,415 or other commercially available precipitated silica powders with fly-ash. This blend of precipitated silica and fly-ash is less expensive than that of precipitated silica alone and is formed into board-like thermal insulating structures for use in thermal insulating panels in a manner similar to that described above. These patents are incorporated herein by reference.
While the precipitated silica powder used alone or blended with fly-ash as described in the aforementioned patents provides satisfactory thermal insulating properties when used in powder evacuated thermal insulating panels, it was found that these precipitated silica powders were essentially non-porous so that thermal conduction through each solid particle detracted from the thermal insulating properties of the powder. Also, it was found that by compacting the precipitated silica or the blend of silica and fly-ash to densities greater than about .018 g/cm.sup.3 resulted in significant solid-solid (particle-particle contact) which increased thermal conduction and thereby decreased the R-value of the thermal insulation.
Recent investigations have shown that the morphology of powders used in powder evacuated thermal insulating panels has a significant impact on the extent of thermal insulation provided by these panels. This level of thermal insulation is commonly expressed as thermal resistivity or R-values in Btu-in/Hr-Ft.sup.2 .degree. F.